1. Field of the Invention
This invention relates to a glass composition for plasma display panel and more particularly, to a glass composition for plasma display panel with the following physical properties of glass, while containing no SrO in principle:
strain point is in the range-of 580.about.610.degree. C.; PA1 transition point is more than 610.degree. C.; PA1 thermal expansion coefficient is 80.about.87.times.10.sup.-7 /.degree.C.; PA1 working temperature, being equivalent to 10,000 poise, is less than 1,200.degree. C.; PA1 Liquidus temperature is less than 1,100.degree. C.; PA1 The viscosity at a liquidus temperature is more than 20,000 poise. PA1 two glass substrates spaced apart from each other by a distance of 100.about.150 .mu.n, each of which thickness is 2.about.3 mm; PA1 the brims closely packed by frit sealing; PA1 the inside of one glass, showing the image, coated with indium tin oxide (ITO) which functions as a data electrode; PA1 the inside of one glass coated address electrode such as Ni, Ag paste and a fluorescent material which serves to develop red, green blue colors. PA1 strain point: more than 600.degree. C.; PA1 thermal expansion coefficient in the range of 0.about.300.degree. C.: 70.about.90.times.10.sup.-7 /.degree.C. PA1 temperature at 10,000.times.10.sup.6 poise: less than 1,240.degree. C.; PA1 viscosity at a liquidus temperature: more than 3,000.times.10.sup.6 poise. PA1 the working temperature is 1,200.degree. C. or less, in spite of the fact that SrO, high-priced chemical raw material, is not contained in principle, PA1 the liquidus temperature is lower than the working temperature, PA1 the viscosity at liquidus temperature is adjusted to 20,000 poise or more. PA1 In spite of the fact that the liquidus temperature is lower than the working temperature, a higher viscosity at a liquidus temperature results in enhancing better productivity; PA1 the glass composition of this invention shows a higher strain point and lower expansion compared to the general sheet glass of soda lime silicate. PA1 the working temperature, being equivalent to 10,000 poise, is less than 1,200.degree. C.; PA1 the liquidus temperature is less than 1,100.degree. C.; PA1 the viscosity value at a liquidus temperature for the molding productivity of glass is more than 20,000 poise; PA1 the strain point for the prevention of a substrate deformation generated from the process of manufacturing the plasma display panel is 580.about.610.degree. C.; PA1 the transition point is more than 610.degree. C.; PA1 the thermal expansion coefficient at 30.about.350.degree. C. is 80.about.87.times.10.sup.-7 /.degree.C.
2. Description of the Prior Art
A plasma display panel belongs to a technology for vacuum fluorescent display, the gists of which are constructed in such a manner that a gas filled between two glass substrates, being maintained at closely packed vacuum state, is ionized by electric discharge and ultraviolet rays are formed. Then, the discharged ultraviolet rays collide with a fluorescent material, being coated within a glass plate, and are transformed into visible rays, thus showing the image in the screen. According to the basic structure of a general plasma display panel, it comprises;
As far as a glass composition for plasma display panel is concerned, a soda lime silicate sheet glass (SiO.sub.2 : 70.about.72 wt %, Al.sub.2 O.sub.3 : 1.about.2 wt %, Na.sub.2 O: 12.about.14 wt %, K.sub.2 O: 0.about.1 wt %, CaO: 8.about.9 wt %, MgO: 4.about.5 wt %) for the use of construction and automobiles, being made available by a float process, was initially applied to a small-size (21") plasma display panel in consideration of some natures of paste or frit used in the process of manufacturing the plasma display panel, such as the thermal expansion coefficient and melting point of the soda lime silicate sheet glass, in particular.
In spite of the fact that demand for a large-size display and high resolution has been increasing, since changes in dimension and flatness and degradation resolution thereof due to shrinkage of the glasses occur in the middle of the multiple firing required in the display fabrication, the soda lime silicate based sheet glass has failed to meet the requirements related to the glass composition for plasma display panel.
The process for manufacturing the plasma display panel is performed at more than 570.degree. C. Since the soda lime silicate based sheet glass has a strain point of 510.about.530.degree. C. (temperature equivalent to 10.sup.14.5 poise as viscosity), a temperature causing the strain of glass on heat, changes in the dimension and flatness of the substrate glass during the multiple firing required in the display fabrication may occur. And these variations in cell gap width lead to variations in the electrical characteristics of the cell, resulting in color variation, in the long run, to dead pixels.
Therefore, it is preferred that the glass composition for plasma display panel has a strain point of more than 570.degree. C. with the thermal expansion coefficient in the range of 80.about.90.times.10.sup.-7 /.degree.C. so as to be suitable for the conventional paste and frit.
The conventional methods designed to manufacture a glass composition for plasma display panel were disclosed in the U.S. Pat. No. 5,459,109 and Japanese UnExamined Patent Publication Hei 3-40933, Hei 8-133778 and Hei 8-165138 and their chemical compositions are shown in the following table 1.
First, the U.S. Pat. No. 5,459,109 based on the float process, a sheet glass based forming technology, had a chemical composition containing no alkaline metal oxide in principle with the following physical properties:
Nevertheless, this patent has recognized some disadvantages in that a) in spite of the fact that the strain point described in the patent claims was more than 600.degree. C., excessively high strain points showing more than 630.degree. C. were described in the examples, b) there was a extremely low thermal expansion coefficient of less than 80.times.10.sup.-7 /.degree.C., and c) even though the liquidus temperature was lower than the working temperature, the viscosity at a liquidus temperature showed less than 15,000 poise, being far lower than 20,000 poise which are desirable in a sheet glass based forming technology. In addition, this patent described that the viscosity values at both working and liquidus temperatures were 10,000.times.10.sup.6 poise and 3,000.times.10.sup.6 poise, respectively, which did not entirely correspond with those of the examples and unreal values that could not be applied in the actual production. Furthermore, as shown in the examples, this patent employed halides (F and Cl) as a fining agent which might cause environmental problems. Further, as shown in the following table 1, a total content of alkaline earth materials such as MgO, CaO, SrO and BaO were in the range of 45.5.about.52.5 wt % and instead of MgO and CaO, natural raw materials, the content of extremely highly-priced SrO and BaO occupied more than 80%. In this respect, any economical benefits are not expected due to the increase in the whole raw material costs. Further, excessive amount of alkaline earth material facilitates the refractory erosion, thus resulting in decreasing the strain point or increasing expansion coefficient, as mentioned in the above. Also, since the B.sub.2 O.sub.3 content, showing 4.5 wt % at maximum was relatively high, its evaporation during the melting process, is expected to cause the refractory erosion in a melting chamber and the refractory erosion in a tin chamber and some glass defects in the process of manufacturing a sheet glass by means of the process.
The Japanese UnExamined Patent Publication Hei 3-40933 disclosed the chemical composition only as shown in the following table 1. Its examples referred to the results of several compositions covering the strain point, softening point, thermal expansion coefficient, electric resistance and thermal deformation amount but there was no description on the sheet glass based forming technology for intended application elsewhere. As far as the claimed ranges of the chemical composition were concerned, a total content of Li.sub.2 O, Na.sub.2 O and K.sub.2 O was described only as alkali oxide and in case of alkaline earth material oxide, a total content of MgO, SrO and BaO including CaO were only described but with the range of CaO specified in more detail. Of 7 among 8 compositions described in the examples, there was no statement on some thermal properties such as the strain point and thermal expansion coefficient related to the glass composition for plasma display panel, compared to a soda lime silicate glass.
The chemical composition of the Japanese UnExamined Patent Publication Hei 8-133778 did not contain ZrO.sub.2 in principle. According to the patent claims, the strain point was more than 560.degree. C. and thermal expansion coefficient in the range of 50.about.300.degree. C. was 80.about.95.times.10.sup.-7 /.degree.C. In addition, its examples referred to the results of various temperatures at 100 and 10,000 poises, liquidus temperature, chemical durability and hardness, while the application of the float process was tacitly described. Instead of containing no ZrO.sub.2 substantially, this patent is intended for improving the hardness of glass with the addition of 0.about.6 wt % ZnO but according to Europe Patent Nos. 559,389 and 510,543 and the U.S. Pat. No. 5,387,560, ZnO has been reported to cause some defects at the surface of a sheet glass, since its reduction in the tin chamber occurs, when a float process, a sheet glass based forming technology is applied. The correlationship between ZrO.sub.2 and ZnO affecting the hardness is quite unclear, when the values on hardness specified in the examples and comparative examples are reviewed, while ZrO.sub.2 has been reported to improve the hardness of glass, on the contrary, according to some literatureChemical Approach to Glass, Milos Volf, Elsevier, 1984, pp. 306.about.314!.
Further, its examples described that though the liquidus temperature was lower than the working temperature, it is nearly impossible to predict any productivity due to the fact that the viscosity at a liquidus temperature was not mentioned elsewhere.
The Japanese UnExamined Patent Publication Hei 8-165138 did contain ZrO.sub.2 as an improved patent of Hei 3-40933 and its examples described the results of thermal properties such as transition point, expansion coefficient, viscosity at high temperature and liquidus viscosity including the application of the float process, a sheet glass based forming technology. However, there was no description on the strain point, being considered as an important factor in the glass composition for plasma display panel and furthermore, there was no description on the viscosity at a liquidus temperature, a criterion of forming productivity when the float process was applied.
TABLE 1 ______________________________________ Japanese Japanese Japanese U.S. UnExamined UnExamined UnExamined Pat. No. Pat. Pub. Hei Pat. Pub. Hei Pat. Pub. Hei Composition 5,459,109 3-40933 8-133788 8-165138 ______________________________________ SiO.sub.2 39.about.43 55.about.65 45.about.66 52.about.62 Al.sub.2 O.sub.3 2.5.about.9.5 5.about.15 0.about.15 5.about.12 B.sub.2 O.sub.3 1.5.about.4.5 -- -- -- Li.sub.2 O -- -- 0.about.0.5 0.about.1 Na.sub.2 O -- -- 0.about.6 0.about.6 K.sub.2 O -- -- 4.about.20 4.about.12 Li.sub.2 O + Na.sub.2 O + -- 6.about.12 10.about.24 7.about.14 K.sub.2 O MgO 0.about.1.5 -- 0.about.6 0.about.4 CaO 0.about.2.5 3.about.12 0.about.14 3.about.5.5 SrO 10.5.about.21.5 -- 1.about.14 6.about.9 BaO 25.5.about.39.0 -- 0.about.14 0.about.13 ZnO -- -- 0.about.6 -- MgO + CaO + 45.5.about.52.5 17.about.25 -- 17.about.27 SrO + BaO MgO + CaO + -- -- 14.about.31 -- SrO + BaO + ZnO ZrO.sub.2 -- 0.6.about.6.0 -- 0.2.about.6.0 SO.sub.3 -- -- -- 0.about.0.6 SO.sub.3 + Sb.sub.2 O.sub.3 -- -- 0.about.1 -- SO.sub.3 + Sb.sub.2 O.sub.3 + -- 0.about.0.5 -- -- As.sub.2 O.sub.3 SnO.sub.2 + TiO.sub.2 -- 0.about.3 -- -- La.sub.2 O.sub.3 -- 0.about.5 -- -- refining agent 0.about.1.5 -- -- -- ______________________________________
In line with such soda lime silicate composition, if the alkali contents such as Na.sub.2 O and K.sub.2 O are reduced so as to increase the strain point and lower BaO, ZrO.sub.2 and Al.sub.2 O.sub.3 are increased the raw materials cost will significantly increase and the overall increases in a temperature related to the melting and molding of glass and liquidus temperature will result in poor productivity of glass. In case of adding B.sub.2 O.sub.3, the melting of glass becomes easier but its increased amount will be responsible for a refractory erosion by evaporation and glass defects thereof. In particular, the addition of ZnO is not preferable in a float process, a sheet glass based forming technology.
With a view to overcoming these defects in the conventional methods, the present inventor et al. carried out extensive studies to fully cope with the float process, a sheet glass based forming technology, by increasing the strain point of glass and reducing the thermal expansion coefficient. In consequence, the present inventor et al. completed this invention to manufacture a glass composition which enables to improve various physical properties as follows: